Multi-level modulation formats, such as quadrature phase shift keying, QPSK, and 16-quadrature amplitude modulation, 16QAM, are widely used for high speed (>40 Gbit/s) transmission in fiber to cope with the bandwidth limitations introduced by optical and electronics devices at the transmitter (modulator driver, optical modulator) and the receiver (photodiodes, trans-impedance and limiting amplifiers). Although a single phase modulator could in principle be used to phase modulate signals in binary phase shift keying, BPSK, QPSK or 8-PSK modulation formats, this is not a practical option due to the high driving voltage required and the narrow modulation bandwidth. IQ modulators, based on two Mach-Zehnder modulators whose optical outputs are π/2 phase shifted relative to each other and then coupled, are commonly used instead to implement these modulation formats. An IQ modulator allows, in principle, an arbitrary set of complex line symbols, modulated in both phase and amplitude (a “constellation”), to be generated. 16QAM is a typical example of a modulation format that cannot be generated using a single phase or amplitude modulator but which can be generated using an IQ modulator driven with multilevel driving signals. However, the need for multi-level driving signals may be problematic, and IQ modulators also face the problems of requiring linear electrical amplifiers, being difficult to realize at high radio-frequencies, and that they may require complicated coding rules to map a sequence of traffic bits of a client signal into a complex constellation symbol.
An optical modulator able to implement QPSK, 8-PSK, 8QAM and 16QAM modulation formats has been reported by Yamazaki, H. (NTT Photonics Labs) et al, “Modulation-level-selectable optical modulator with a hybrid configuration of silica PLCs and LiNbO3 phase modulators”, ECOC, 19-23 Sep. 2010, which comprises a lattice configuration of Mach-Zehnder modulators and enables modulation-level switching (from 4, 8 and 16 levels) using tunable optical couplers to change the number of Mach-Zehnder modulators that contribute to the modulation.